1. Field
This invention relates to process indicators for use with articles which are periodically sterilized. It is specifically directed to a composition which may be printed on substrates and which changes from an initial color to a second color after exposure to a selected sterilizing temperature.
2. State of the Art
Process indicators for monitoring sterilizing procedures typically rely upon color changes to verify that the items which have been processed have in fact been subjected to a sterilizing temperature. Available process indicators of the type intended to verify heat history, have typically come in two types. Certain such indicators rely upon chemical changes. These indicators verify only that the indicator has been exposed to a particular temperature. They are not informative concerning the duration of exposure of the indicator to that temperature. In practice, sterilization requires exposure to a sterilizing temperature for a prescribed length of time. A second class of indicators relies upon a physical change of some type; e.g., melting wax or dissolution of one substance into another substance. Representative of patents which disclose process indicators of various types are U.S. Pat. Nos. 2,118,144 which teaches reliance upon a chemical change (sulfur and lead carbonate); 2,606,654 which suggests the use of heat unstable dyes ("fugitive dyes") 3,360,338; 3,386,807; and 3,684,737 which disclose indicators dependent upon chemical changes of various types; 3,946,612 which discloses a physical change corresponding to the action of a solvent; and 3,114,349 wherein a substance is disclosed which changes hue as one of its components melts.
There are a multitude of process indicators commonly used in the health care field. Many of these comprise chemical formulations containing ink. The formulation is applied to paper, cardboard or plastic substrates in the form of bags, tapes or other forms of packaging or indicator strips. A more complex type of indicator includes a colored substance blended with wax encased in an ampoule. As the wax melts, the ampoule displays a color change. Because of certain limitations in the indicators, notably the fact that the color change occurs without regard to other sterilization parameters, such as duration of exposure to a prescribed temperature, the indicators are relied upon primarily as an aid in preventing the reuse of items which may not have been put through the sterilizer at all. They are not truly a "fail safe" check on sterilization.
Among the problems associated with the heat history indicators of the prior art are the possibility of chemical contamination through the introduction of potentially harmful chemicals into the sterilizer chamber; limited shelf life, because of the tendency of many of the chemical compounds relied upon in the prior art to react upon exposure to light and/or heat substantially below sterilization temperature; difficulty of application of the indicator to the substrates desired for use in sterilizers, particularly in connection with printing the substrate with an indicator in ink form; and the display of a psychologically inappropriate color at the end of the sterilizing cycle. With respect to color change, most technicians experience some difficulty in recognizing such colors as brown or red as an indication that an article has been sterilized. There apparently exists no heat history indicator within the prior art which changes from a color readily recognized as a caution color (such as yellow, red or orange) to a color readily recognized as a safe or "proceed: color (such as green).